Analog and video sources can vary widely in the signal amplitudes they produce. Receivers and input amplifier circuits, however, are limited in the ranges of voltages and currents they can process. So a strong input signal in the first stages of amplification with the gain set too high can result in saturation and clipping, the output will therefore be highly distorted and not very useful. Too little amplification on a faint signal may also produce an output that is not useful.
Automatic gain control (AGC) circuits were first widely used in television and radio receivers where the channels being tuned can vary widely in strength. AGC allows the audio volumes and video contrasts to be presented at consistent levels. Audio AGC has also been employed recently to maintain a consistent volume between programs and commercials, and in Internet telephony applications. Originally, purely analog AGC methods were used, but now digital circuits allow the implementation of digital automatic gain control (DAGC).
Jum-Han Bae in U.S. Pat. No. 6,195,133, issued Feb. 27, 2001, and Pascal Audinot, et al., in U.S. patent application Ser. No. US 2004/0259510, published Dec. 23, 2004, describe DAGC implementations that necessitate or involve the use of A/D converters. A/D converters tend to increase the costs and increase AGC settling times. While an AGC circuit hunts for the right level and the controlled amplifiers settle in to these new levels, the signal outputs can be distorted or otherwise useless, and such will limit the applications for which the AGC circuit can be employed.
In view of this, what is needed is a DAGC system that is simple, inexpensive to manufacture, and that settles quickly.